Hundreds of Europeans who fought alongside ISIS in Syria and Iraq have returned to a continent officials say lacks the resources to monitor them all.

The returning fighters are an increasing challenge to security and intelligence officials who believe recent terror attacks in Paris and Brussels are part of a continuing effort by ISIS to keep launching deadly attacks inside Europe, officials told BuzzFeed News.

Between 4,000 to 6,000 people from European countries have traveled to fight in Syria, Iraq, and Libya, and roughly 10% are believed to have returned, many bringing with them deadly expertise in weapons, explosives, and an extremist ideology that has already been spreading within European borders.

"Those people that have been traveling there two, three, four years ago, they are absolutely trained and ready to fight," French Senator Nathalie Goulet, who co-heads a commission to track jihadis, told BuzzFeed News. "For us, it's a new threat."

Law enforcement agencies across Europe not only lack the ability to keep tabs on all returning fighters, which Goulet estimates to be between 400 and 600, but poor intelligence sharing between European nations is hindering efforts address the threat of ISIS operatives, she said.

"Turkey said they send information to Belgium, and nobody paid attention to it," she said. "We have to have more cooperation."

Most of the returning fighters were likely not "operatives" under the direction of ISIS, but the terrorist group appears engaged in a campaign to continue deadly attacks inside Europe, Malcolm Nance, executive director of Terror Asymmetrics Project and a former U.S. counterterrorism officer, told BuzzFeed News.

Some of them might have been wounded, grown tired of the battlefield, or even become disillusioned with ISIS. Still, the bulk of fighters returning home pose a challenge to intelligence agencies there, Nance said.

"[French] intelligence about who is in their country is very damn good," he told BuzzFeed News. "That doesn't mean they can track everyone on the ground."

Goulet, the French senator, agreed.

Salah Abdeslam, who is suspected of participating in the Paris attacks in November before returning to Belgium, was captured just a few days before the attacks in Brussels were carried out.

"I'm not confident at all," she said about the ability of France and other countries to keep track of returning fighters. "For the time being, I'm very anxious."

Charles Platiau / Reuters

Nance said the number of ISIS operatives in Europe is probably around 70, though their capabilities vary. But a cell of 20 or 30 individuals could still cause havoc, he said.

And terror plots on European soil are likely to continue, Seth Jones, director of the International Security and Defense Policy Center at the RAND Corporation, told BuzzFeed News.

"We'll probably see more of this," he said. "[ISIS] is both leveraging resources they have in Europe and targeting European countries, which they consider infidels."

With ISIS continuing to lose territory in Iraq and Syria, the militant group has shifted resources to external attacks, Jones said. A loss in territory means manpower is freed, and leaders might focus their anger inside Europe instead.

The flow of fighters who have traveled from Europe to Syria, Iraq, and Libya is larger than what has been seen in previous conflicts, and their return could mean many of them come back with a directive or self ambition to launch an attack, Jones said.

Goulet had no specific information about ISIS targeting Europe, but said France and its neighbors remain on edge.

"All governments know to put the threat level very, very high," she said. "There is no question we are in a very high level of threat."

It's a state that experts said is likely to continue for some time.

"The threat levels we are seeing in Europe will likely remain very high for the foreseeable future," Jones said. "They have more people to monitor than they have people to be able to monitor them with."